Ever since I had multiple users on my Nexus 7 I wanted a guest user on my phone. To try software I don’t really trust and to hand to others to go on the internet etc.
I recently found this: How to enable multiple user mode on CyanogenMod 10.1
Of course I had to try it and lo-and behold. A guest user on my phone. I’ve had issues accepting a phone call the other day so YMMV on how useful this is, but all in all I like the idea :)